Bolt Torque Calculator (ISO Metric)
Bolt torque calculator for ISO metric coarse-thread bolts, M6 to M36. T = K·F·d with published nut factors from Fastenal, Fastener Facts and NASA RP-1228, or your own K.
Bolt Torque
Coarse thread only. Fine-pitch threads of the same diameter (M12 × 1.25, for example) have a larger stress area, and this tool does not carry those areas.
The sources do not agree on K. For zinc-plated bolts Fastenal gives 0.17 and the Fastener Facts table 0.20; for "lubricated", 0.15 and 0.18. A worked example from Bossard gives electro zinc plated bolts a friction coefficient of 0.14 to 0.24 (a friction coefficient, not K). If your fastener supplier publishes a K for the parts you have, enter it as your own.
ISO 898-1 gives class 8.8 two proof stresses, split at 16 mm: 580 MPa at or below M16 and 600 MPa above it. The other classes here are single-valued. The one documented exception is structural bolting, which takes 600 MPa from M12 upward — select it above only if that is what you are designing.
| Nut factor K | — |
| Bolt nominal diameter d | — |
| Stress area A_s | — |
| Proof stress S_p (ISO 898-1) | — |
| Proof load F_p | — |
| Target preload F (75% of proof) | — |
This is arithmetic, not a specification. The torque gives the target preload only if the real K equals the K used. Where the real K is lower, the same torque clamps harder than the target; above 100% of proof load a bolt can yield.
How to use the bolt torque calculator
Pick bolt size and property class
Sizes are ISO metric coarse thread, M6 to M36, with the pitch shown beside each. The property class is the number marked on the bolt head. The tool takes the proof stress for each class and the stress area for each size from ISO 898-1.
Choose a nut factor K
Every K in the list is a published value, grouped under the table it comes from. The tables disagree, so the same words, such as "zinc plated" or "lubricated", appear more than once with different values. Choose Enter your own K to use a value from your fastener supplier or a test.
Set the preload
As a percentage of proof load, above 0 and no more than 100. The default is 75%, the level Fastenal's chart uses for its clamp loads. Values outside that range are refused rather than adjusted.
Read the torque, then the range
The torque is T = K × F × d at the K you chose. Below it the tool shows the clamp load the same torque gives at the lowest and highest published K in the list, and says when that would pass 100% of proof load. A torque the maker of a part publishes for that joint is its specification; this calculation is not.
Bolt torque and the nut factor K
The short-form relation between tightening torque and the clamping force in a bolt (its preload) is T = K × F × d: torque equals the nut factor K, times the preload F, times the nominal diameter d. K is not a property of the steel. It lumps together the friction in the threads and the friction under the head or nut, so it is looked up or measured, and it changes with finish, coating and lubricant.
"Torque is only an indirect indication of tension." — Fastenal, torque-tension chart for ISO 898-1 metric fasteners
What the published nut factors say
Fastenal's charts use K 0.20 for plain and dry bolts, 0.17 for zinc plated and dry, and 0.15 for "lubricated" conditions. A four-row table credited to Bowman Distribution's Fastener Facts (1985), and reproduced in SOLIDWORKS Help and in Shigley's textbook, gives 0.30 for non-plated black finish, 0.20 zinc-plated, 0.18 lubricated and 0.16 cadmium-plated, and recommends 0.20 when the bolt condition is unknown. NASA's fastener design manual, RP-1228 (1990), calls 0.2 "the commonly assumed value", warns it "should not be used blindly", and gives 0.15 as a more realistic typical value for steel on steel.
The published values in this tool run from 0.15 to 0.30. Because the clamp load at a fixed torque is T ÷ (K × d), the same torque clamps twice as hard at K 0.15 as at K 0.30. That is the range the results panel shows for every answer.
How far preload scatters under torque control
NASA RP-1228 reproduces two tables of preload accuracy and notes that their numbers vary. The Industrial Fasteners Institute table rates a torque wrench at ±25%, turn of the nut ±15%, load-indicating washers ±10%, fastener elongation ±3 to 5% and strain gages ±1%. A 1981 Machine Design table rates torque control at ±15 to ±30%, turn control at ±15 to ±30%, torque and turn together at ±10 to ±25%, and bolt stretch at ±1 to ±8%.
Preload and vibration
NASA RP-1228 says higher torques, up to theoretical yield, are sometimes used for bolts that cannot be locked against vibration: "The higher load will increase the vibration resistance of the bolt", but the bolt "will yield and unload" if its yield point is exceeded.
ISO property classes and proof load
The proof stresses here are the ISO 898-1 values: 225 MPa for class 4.6, 380 for 5.8, 580 for 8.8 up to M16 and 600 above, 830 for 10.9 and 970 for 12.9. Multiplied by the coarse-thread stress areas, the values for 4.6, 8.8, 10.9 and 12.9 reproduce the minimum proof loads printed on Fastenal's ISO 898-1 chart; for M12 class 8.8 that is 48.9 kN. The tool's M12 class 8.8 answer at K 0.20 and 75% preload, 88.0 N·m, matches the dry torque that chart prints.
Nut factors from Fastenal, Fastener Facts and NASA, and how far preload scatters
T = K × F × d: torque = nut factor × preload × nominal diameter.
Fastenal: K 0.20 plain and dry, 0.17 zinc plated and dry, 0.15 lubricated.
Fastener Facts (Bowman, 1985): 0.30 non-plated black finish, 0.20 zinc-plated, 0.18 lubricated, 0.16 cadmium-plated.
NASA RP-1228: 0.2 is "commonly assumed"; 0.15 is a more realistic typical value for steel on steel.
At one torque, K 0.15 gives twice the clamp load of K 0.30.
ISO 898-1 class 8.8: proof stress splits at 16 mm, 580 MPa to M16 and 600 MPa above.
Industrial Fasteners Institute, via NASA: torque wrench ±25% preload accuracy; strain gages ±1%.
Machine Design, via NASA: torque control and turn control both ±15 to ±30%; bolt stretch ±1 to ±8%.
NASA: higher preload increases vibration resistance, but a bolt taken past yield unloads.
Fastenal's metric chart computes its clamp loads at 75% of proof load.
Frequently asked questions
A dimensionless number that lumps thread friction and the friction under the head or nut into one factor in T = K × F × d. It is looked up or measured, not calculated from the bolt's strength. The values in this tool come from three published sources, which do not agree with one another.
A label such as "zinc plated" covers a spread of real surfaces. Fastenal gives zinc plated and dry 0.17; the Fastener Facts table gives zinc-plated 0.20. A worked example in Bossard's preload and tightening-torque guide gives electro zinc plated bolts a friction coefficient of 0.14 to 0.24, which is a friction coefficient rather than K, but shows how wide one label can be. That is why the tool names the source of every K and accepts your own.
NASA RP-1228 reproduces two answers. The Industrial Fasteners Institute rates a torque wrench at ±25% preload accuracy; a 1981 Machine Design table rates torque control at ±15 to ±30%. Both rate direct measurement much tighter: strain gages ±1% (IFI), bolt stretch ±1 to ±8% (Machine Design).
No. It is this tool's default and the level Fastenal's chart uses for its clamp loads. The tool accepts any preload above 0 and up to 100% of proof load. The torque it returns reaches that preload only at the K you chose: at a lower real K the same torque clamps harder, and above 100% of proof load a bolt can yield.
NASA RP-1228 says more: "The higher load will increase the vibration resistance of the bolt". It adds that a bolt will "yield and unload" if its yield point is exceeded, and that the exact yield torque cannot be found without extensive instrumentation.
The joint is first tightened to a snug torque, then turned through a set angle. It is not independent of friction: the snug step is itself a torque. The two tables in NASA RP-1228 disagree on how much it gains. Machine Design rates turn control at ±15 to ±30% preload accuracy, the same band as torque control, while the Industrial Fasteners Institute rates turn of the nut at ±15% against ±25% for a torque wrench.
No. Every size is ISO metric coarse thread, with the pitch shown in the size list. A fine-pitch thread of the same diameter has a larger stress area, so a coarse-thread result does not carry over to it.
The relation T = K × F × d is the same. This tool covers ISO metric property classes only; inch grades such as SAE Grade 5 and 8 have their own proof loads, which Fastenal publishes in a separate chart.
No. All inputs stay in your browser.
NASA's Fastener Design Manual, RP-1228; Fastenal's metric and inch torque-tension charts; SOLIDWORKS Help on the friction factor; and Bossard's preload and tightening torque tables, which work from VDI 2230.
Method & sources
How it computes
T = K·F·d: F is the target preload, ISO 898-1 proof stress Sp × tensile stress area As (ISO metric coarse, M6-M36) × your preload % (above 0, at most 100% of proof; default 75%); d is the nominal diameter. K is a published nut factor named with its source (Fastenal 0.20 / 0.17 / 0.15; Fastener Facts 0.30 / 0.20 / 0.18 / 0.16; NASA RP-1228 0.15) or your own. The result also shows the clamp load the same torque gives at K 0.15 and 0.30.
What this tool implements
- ISO 898-1:2013 stress under proof load Sp: 225 MPa (4.6), 380 (5.8), 580 (8.8, d <= 16 mm), 600 (8.8, d > 16 mm, and for structural bolting from M12), 830 (10.9), 970 (12.9). With the coarse-thread stress areas these reproduce the minimum proof loads on Fastenal's ISO 898-1 chart for 4.6, 8.8, 10.9 and 12.9 (M12 8.8: 48,894 N; M16 91,060 N; M18 115,200 N, so the 16 mm split).
- ISO metric coarse thread only; pitches as Fastenal's metric chart prints them (M12 x 1.75). Fine pitch is not offered: no published table of fine-pitch stress areas was opened.
- Nut factors as published, each labelled with its source: Fastenal "K = 0.20 for plain and dry conditions", "K = 0.17 for zinc plated and dry conditions", "0.15 used for "lubricated" conditions"; Fastener Facts (Bowman 1985, p. 90, via SOLIDWORKS Help) non-plated black finish 0.30, zinc-plated 0.20, lubricated 0.18, cadmium-plated 0.16, and "If the bolt condition is unknown, K = 0.20 is recommended"; NASA RP-1228 "commonly assumed" 0.2 and 0.15 "typical" for steel on steel. An unknown condition falls back to 0.20.
- Preload scatter as NASA RP-1228 reproduces it: Industrial Fasteners Institute torque wrench +/-25%, turn of the nut +/-15%, strain gages +/-1%; Machine Design (1981) torque control +/-15 to +/-30%, turn control +/-15 to +/-30%, bolt stretch +/-1 to +/-8%. NASA: higher preload "will increase the vibration resistance of the bolt".
- The 75% default is the level Fastenal's chart computes its clamp loads at ("Clamp load calculated as 75% of the proof load"), not a requirement; preload is a user input, refused at or below 0 and above 100%.
- Corrected 2026-10-19: K 0.30 labelled zinc-plated (a black-finish value; M12 8.8 "zinc" 75% returned 132.0 N-m, 132% of proof at Fastenal's 0.17); unsourced "lubricated" K 0.10; preload unbounded (0, negative and 200% accepted); "50% preload for vibration-sensitive joints" (NASA says higher preload resists vibration); angle control "independent of friction"; three different scatter figures; unsourced relaxation, re-torque, wrench-accuracy and "12.9 for aerospace" claims; Shigley Table 8-15 cited for values that contradicted it.
Sources
- Barrett RT. Fastener Design Manual. NASA Reference Publication 1228. Cleveland, OH: NASA Lewis Research Center; 1990. Tables VI-VIII. https://ntrs.nasa.gov/citations/19900009424
- Fastenal. Torque-Tension Relationship, ISO 898-1 Property Class 4.6, 8.8, 10.9 & 12.9, Metric Coarse Thread. Jan 2016. https://crafter.fastenal.com/static-assets/pdfs/technical-resourc…
- Fastenal. Torque-Tension Relationship for A307A, Grade 5, 8 & 9 Bolts. Rev 3-6-09. https://crafter.fastenal.com/static-assets/pdfs/Torque-Tension_Ch…
- Dassault Systemes. SOLIDWORKS Help 2018: Definition of Friction Factor (table credited to Bowman Distribution-Barnes Group, Fastener Facts, Cleveland, 1985, p. 90). https://help.solidworks.com/2018/english/SolidWorks/cworks/r_Fric…
- Bossard. Preload and tightening torques (VDI 2230 tables and worked example). 01-2025. https://www.bossard.com/sg-en/knowledge-hub/resources/technical-i…
- ISO 898-1:2013. Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes — Coarse thread and fine pitch thread. Geneva: ISO; 2013. https://www.iso.org/standard/60610.html
What can make this go out of date
- ISO 898-1 is revised periodically (current edition 2013); Sp and As move only with a new edition.
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